Mirror Cleaning

Steward Observatory on Mount Lemmon

This procedure for cleaning large mirrors was demonstrated to us by Joseph Hochscheidt when he cleaned the Manner Telescope primary and secondary in June 2018. Use it with caution since this is based on notes taken at the time. Obviously the first rule is do no harm, and that means be reasonable, gentle, and follow the advice of experts when it is available.

For this cleaning process the mirror was left in the telescope. It was a warm sunny day, and the telescope enclosure was opened slightly to allow for ventilation and to provide illumination needed to judge the effectiveness of the process.

1. Blow off all dust and loose particles with dry nitrogen gas from a compressed gas cylinder.

This requires bringing a cylinder to the telescope, attaching a regulator, and using a flexible hose that provides enough gas flow for the purpose. Do not use compressed air! It will contain moisture that will condense on the optics, and it may also contain fine particulates that could damage the coatings. Also do not use dust-off gas often sold for blowing dust out of electronics and even for photographic lenses. Typically these contain fluorocarbons and they will leave a residue that is hard to remove. An alternative that is often used with large optics is to use gas withdrawal carbon dioxide. This is available in small cylinders, and is released at full pressure through a hose to direct the flow onto the mirror and its shrouds. Carbon dioxide cleaning will produce CO₂ snow which can effectively scrub a mirror of dust. However it will be insufficient to remove deposits left by evaporating dew or by insects. If a supply of dry nitrogen or carbon dioxide is not available the surfaces may be dusted with a new very clean and soft brush. A photographic lens camel's hair brush, or a soft paint brush with fine bristles will be safe for this task. Compressed gas is preferred, however.

Assure that all loose material is blown out or wiped from the housing that baffles the mirror.

2. With the telescope pointed approximately horizontally, use large quantities of Kimwipes or paper towels rolled to absorb any fluids that drain from the mirror. These may be replaced several times during the cleaning process. The purpose is to prevent fluids from draining into critical components behind the primary, or auxiliary instruments or components that may be on the lower side of the truss. Each case will be unique and should be evaluated before beginning. It may be beneficial to point the telescope slightly below the horizontal if the light baffle around the primary is solid and would hold liquid that could drain to an area which is not critical.

It is not advisable to remove the primary. This adds considerable risk to the cleaning process, and would surely require telescope realignment. Also, primary removal would normally be preceded by mechanical work to assure the telescope remains stable, and would add days to a job that can be done in hours.

3. Wash the interior and all exposed surfaces with water.

4. Prepare a solution of distilled water and detergent. The distilled water can be purchased at a grocery or drug store in gallon jugs. It should be mixed in a clean 5 gallon bucket reserved only for mirror washing. For mirrors of the order of 0.5 meter in diameter, 2 gallons of water for the washing solution should be adequate. Add a small quantity of Liquinox detergent to the water. Test the solution to see that it wets a surface easily. Excess detergent leaves a residue that requires more rinsing and the compromise needed is to have enough in solution to clean, and not so much it is difficult to rinse. Only a few milliliters of liquid detergent should be needed in 2 gallons (about 8 liters) of distilled water. If Liquinox is not available, then Dove dishwashing liquid also works well.

5. Prepare suitable washing wipes. At Steward the preferred ones are treated sea sponges that have been soaked in vinegar to remove minerals and then very thoroughly washed and rinsed. The final wash and rinse is done with TX609 Technicloth wipes bundled to make a poof ball perhaps 10 cm in diameter and held by the back where the edges of the cloth are gathered. You will need many of these and it helps to have an assistant making them as you proceed. At this stage both the person doing the mirror cleaning and the assistant should have on new clean nitrile gloves so that skin oils are not collected and left on the mirror.

6. Apply the water and detergent mix to the mirror working in thirds. It may take two or more treatments to achieve a clean surface, and a preliminary rinse should be done on the work area before moving to the next third.

7. Rinse twice with distilled water and pat dry with a TX609 wipe poof. The rinse should be using a separate rinse bucket of water into which to dip the wipe. The final rinse is with a wet but not dripping wipe. The goal is not to leave water droplets on the surface, and not to pick up debris into the wipe that will scratch the coating. By using a large clean bucket and fresh wipes on each pass, you are assured that contamination of the water and the wipe is minimized. When you dip a previously used wipe into distilled water you will contaminate the water with residual detergent and you risk contaminating with particulates that were picked up off the mirror. A final rinse with a second filling of distilled water will give a better result than making do with one rinse and dipping back into he water several times.

The primary mirror of the 0.6-meter Manner Telescope on Mt. Lemmon during cleaning. Packing around the lower edge absorbs cleaning fluids. The upper right third of this mirror has been cleaned and washed. The lower portion has not been cleaned for two years.

Materials required:

• Nitrile gloves
• Dust off or cleaning gas such as
  • Prepure N₂ tank with regulator and hose
  • CO₂ tank with a siphon tube (liquid withdrawal) low velocity snow generator (see below)
• Bounty paper towels for handling excess liquid and runoff
• White Kleenex tissue free of fragrances and oils for spot cleaning
• Texwipes as a technical alternative to off-the-shelf paper towels (caution, these will scratch some coatings)
• TX609 Technicloth for direct use on mirror coatings during the wash and rinse stages
• Soft brush such as a very high quality new fine bristle paint brush or a camera lens brush (these collect oils and abrasive dust; best not to reuse)
• Isopropyl alcohol 90% for removing stubborn organic contaminants
• Distilled water (several gallons)
• Clean 5 gallon buckets
• Detergent
  • Liquinox
  • Dove dishwashing liquid (alternative to Liquinox)

Suppliers:

• Disposable nitrile gloves are available from all drugstores and most grocery stores for medical purposes. Use the dust-free variety. Replace them frequently if you are handling the material that will be in contact with the mirror surface.
• Distilled water is available off the shelf in drug and grocery stores. You will need several gallons. Pour a supply into a clean bucket and let it settle. Use the liquid at the top of the bucket
• Buckets. Start with new clean plastic 5-gallon buckets and label them for wash and rinse liquids. After a first use rinse with distilled water and store covered for reuse on the next cleaning. Do not use buckets that have had other uses. New buckets are available from home suppliers (Home Depot), hardware, and paint stores.
• CO₂ snow cleaning equipment
  • Carbon Dioxide Snow Cleaning by Applied Surface Technologies]
  • The Subaru Telescope cleaning snow process
• N₂ or CO₂ gas cylinders
  • Local welding gas suppliers will refill tanks
• Texwipes
  • Amazon search for texwipes and then for TX609 to have the lint free version
• Liquinox
  • Amazon links to suppliers for this too. A small amount of Alconox will do.
• Isoproponal
  • Isopropyl alcohol is a readily available substitute for pure ethanol to be used as a cleaning agent to remove oils and to hasten drying. It is available off the shelf in drug and grocery stores. Use the 90% pure antiseptic medical alcohol in a new previously unopened bottle. Alcohol will adsorb water from the air once opened.